Unfortunately, I've just discovered that I can't take full advantage of the cloud storage capabilities of Evernote, because the AMR files won't play in Firefox (or Chrome). This problem is discussed in the Evernote forums, though the information there is a little out of date.

At first I was getting a "missing plugin" error. However, since then I have installed the Medibuntu repository, VLC player, and libopencore-amrnb0 version 0.1.3-2. Now, when I open an Evernote audio note in Firefox, I see this interface:

While the plugin image seems promising, sadly, when I hit the play button (or right click and select "Play"), nothing happens.

I have also now discovered that if I download the AMR file to my computer, and open it with VLC, it plays. It also plays in Totem and Mplayer. So the problem seems to be specific to playing AMR files within the browser.

Why would playing an AMR file using the VLC plugin in a browser be different from using the stand alone player? More importantly, how can I get Ubuntu to play AMR file directly in Firefox in Ubuntu?

Totem can indeed play amr files. Playing audio files from evernote feature works fine in chrome, for me, as well as in NixNote(an Evernote clone). I've used mozplugger to change how firefox handles mime type, and I can get totem to attempt to open the file. However I get a permissions error and the amr file fails to play.
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Kevin BowenDec 19 '12 at 5:52

Thank you for your answer, but I think you missed my update to my question. I can see the Totem plugin in Firefox, but it is failing to play my AMR files.
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QuestionerDec 19 '12 at 6:49

@DaveMG i was just showing you an alternative to totem.
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Mevin BabuDec 19 '12 at 7:03

Oh, sorry, I got confused. I think it's VLC that's showing in my Firefox, and I was calling it Totem. So... just to be clear... it's VLC in Firefox that is not playing AMR files.
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QuestionerDec 19 '12 at 9:29

As per my updated question, it turns out VLC, and other players, can play AMR files when downloaded to the computer. They only fail when played via plugin in the browser. So the question has become, why does VLC fail when it's a plugin?
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QuestionerDec 26 '12 at 16:55

The extension Adblock Plus has an interesting feature that allows you to "block" any embedded media you'd like. But in order to do that, it will tell you the full URL of that media file.

What I do is install ABP on Firefox.

On EverNote, mouse-over the "missing plugin" window, and a "Block" button will show up.

Click on the button and it will display the full URL of the AMR file.

Then simply copy/paste that URL in Firefox and the browser will allow you to work with it as a regular downloadable file. To save a few more seconds, simply set your Firefox to automatically open AMR files with VLC or Totem.

It does take a little fiddling, but once installed the process to open an audio note takes only seconds of copy and paste a URL.